Butted spokes are lighter and stronger than a traditional straight gauge spoke. When the metal is manipulated (stretched) it work hardens and becomes stiffer. Since there is less in the center of the spoke it is lighter as well. On a bicycle it can make a world of difference (we used to use double butted spokes as well), but on a motorcycle I am not sure you are even going to be able to tell. One thing to note is that once a spoke is butted it is not as easy to true as a single gauge spoke, since the metal is stiffer. So on one hand you have the lighter wheel set with a butted spoke, but on the other hand if you need to true it, or put it in round, a single gauge spoke is going to do better.
I used to build bicycle wheels for professional mountain and road bike teams. If you have any other questions please feel free to ask.